A boosted engine may offer greater fuel efficiency and lower emissions than a naturally aspirated engine of similar power. During transient conditions, however, the power, fuel efficiency, and emissions-control performance of a boosted engine may suffer. Such transient conditions may include rapidly increasing or decreasing engine load, engine speed, or mass air flow. For example, when the engine load increases rapidly, a turbocharger compressor may require increased torque to deliver an increased air flow. Such torque may not be available, however, if the turbine that drives the compressor is not fully spun up. As a result, an undesirable power lag may occur before the intake air flow builds to the required level.
It has been recognized previously that a turbocharged engine system may be adapted to provide “blow-through” air wherein boosted intake air is driven from the intake manifold, downstream of the compressor, directly into the exhaust manifold, upstream of the turbine. For example, a variable cam timing may be temporarily adjusted to a timing that provides high valve overlap. During the positive valve overlap, the boosted air is inducted through the cylinders into the turbine to temporarily provide extra mass flow and enthalpy in the exhaust. The extra turbine energy enables the turbine to spin-up faster reducing turbo lag.
However, the inventors herein have identified potential issues with such an approach. As one example, the valve timing corresponding to the high valve overlap position may not be a timing that is optimal for engine performance. That is, the valve timing may have to be temporarily moved away from a desired timing. Consequently, the high valve overlap position may degrade brake specific fuel consumption (BSFC) as well as the indicated engine torque production. As such, this degrades engine fuel economy and performance. As another example, a fast response of the variable cam timing (VCT) device is required to transiently move to the blow-through (or high valve overlap) position and then return to the regular timing (with reduced valve overlap). If the required response time is faster than can be provided by the VCT device, engine performance and fuel economy may be further degraded. Further still, in order to provide the blow-through, the engine has to be in a positive pumping regime (that is, under boosted engine operation), else may occur in an opposite direction to that desired, degrading turbocharger performance.
Thus, at least some of the above issues may be addressed by a method for a turbocharged engine. In one embodiment, the method comprises, directing intake air from an intake manifold, downstream of a compressor, to an exhaust manifold, upstream of a turbine via each of external EGR and positive valve overlap through a cylinder, wherein amounts of air directed via EGR and air directed via positive valve overlap are adjusted based on conditions. In this way, intake air can be directed to the exhaust manifold to rapidly increase exhaust pressure without degrading engine performance.
As an example, in response to a tip-in, a controller may determine an amount of blow-through air to be directed from the intake manifold to the exhaust manifold to expedite turbine spin-up. The determined amount of blow-through air may then be delivered via the cylinders if a cylinder intake and/or valve timing can be adjusted from a current timing (e.g., a timing corresponding to negative valve overlap) to a timing that enables positive valve overlap without degrading combustion stability. For example, the intake and/or exhaust valves may be operated via a variable cam timing (VCT) device and it may be determined if the VCT adjustment required to place the valves at a valve timing that provides positive valve overlap is within a pre-defined range. As such, outside this range, engine torque output may be affected and/or combustion stability may be degraded. If the required VCT adjustment is within the range, then intake and/or valve timings may be adjusted to temporarily provide positive valve overlap, and intake air may be directed from the intake manifold to the exhaust manifold via the cylinders during the positive valve overlap.
If, however, the required VCT adjustment would degrade combustion stability and engine torque output, at least a portion of the blow-through air may be provided via an EGR passage coupled between the intake and exhaust manifolds. For example, an EGR valve may be opened while no VCT adjustment is performed to direct the entire amount of blow-through air via the EGR passage. Alternatively, a smaller VCT adjustment (that is, within the desired range) may be performed so that a portion of the blow-through air is provided via a smaller amount of positive valve overlap, while a remaining portion of the blow-through air is provided via the EGR passage. In each case, a fuel injection amount may be adjusted during the directing of blow-through air via positive valve overlap and/or EGR based on the amount of blow-through air so that an exhaust air-to-fuel ratio is maintained substantially at stoichiometry.
In this way, extra mass flow and enthalpy may be provided in the exhaust to expedite turbine spin-up and reduce turbo lag without degrading engine performance. By providing at least a portion of blow-through air via an EGR passage, turbo lag may be addressed while valve timing is retained at a timing that improves engine performance. By using positive valve overlap to direct the blow-through air only when there is no fuel penalty associated with a VCT adjustment, engine fuel economy is also improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.